Luminescence

Thermoluminescence uses heat to release already-excited ions in a solid. When subjected to ultraviolet light, x rays, or gamma rays (all of which are energetic enough to separate electrons from atoms, thus forming ions), some electrons or ions become trapped in excited states. They are prevented from decaying back to a ground state because quantum mechanics forbids the transition. Heating allows the ion to rise to a higher state that can drop back to ground state by emitting light.

Thermoluminescence can be used to measure how much radiation a material has been subjected to. It is used for dosimeters by people working around x rays or radioactivity who need to know how much ionizing radiation they have been exposed to. Thermoluminescence is also used for radioactive dating of pottery shards and for finding radioactive minerals.